Future work on age dating

The upcoming papers will analyze the remaining galaxies in our dataset (in total 11 galaxies). This will allow us to discuss the results in the light of galaxy properties. In particular, we will look at trends with galaxy size and environment. The local density (Tully 1988) covered by our galaxy sample spans from 0.08 Mpc−3 (NGC 3115) to 4.17 Mpc−3(M87) and environmental effects appear to be an important ingredient to galaxy formation and evolution.

On the modeling side, our goal is to improve the quantitative information of our method. We are developing a χ2_{-test to find the best solution in the two parameter space of the models: ratio}

old/young and age of the young population, which are slightly degenerate. We also plan, with the help of spectroscopy and wide field photometry, to be able to better calibrate the models in terms of absolute age.

Also, we are exploring the dependence of the results on a particular SSP model and we will repeat the determination of the age structure using SSP models by Maraston (2000) and Vazdekis (1999). Further, we will investigate in more detail the effect of background contamination. This latter aspects will be the subject of a separate paper.

The authors would like to thank the ESO user support group and the ESO science operations for having carried out the program in service mode. We are also grateful to Stephane Charlot for providing his population synthesis models prior to publication. M. Hilker acknowledges support through Proyecto Fondecyt 3980032. THP gratefully acknowledges the support by the German

Deutsche Forschungsgemeinschaft, DFG project number Be 1091/10–2. DM is supported by FON- DAP 15010003 Center for Astrophysics.

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4 Intermediate-age Globular Clusters in early-type

Galaxies

Better age determinations by adding U-band

observations to the V,I,K datasets

Astronomy & Astrophysics, accepted

Abstract

This paper represents an extension to a series of publications on combined optical and near-infrared photometry of globular cluster systems in early-type galaxies. Based on color-color diagrams the cumulative age distributions of their globular cluster systems have been derived and revealed in- termediate age clusters in two galaxies, namely NGC 4365 and NGC 5846. The extension of this observations towards the blue wavelength range was performed in order to increase the age res- olution of photometric studies. In this paper we present the results of U-band observations of NGC 4365 and NGC 5846 and their combination with previously obtainedV, I and Ks- band pho-

tometry. (U−I)vs.(V−Ks) color-color diagrams are used to derive the cumulative age distribution.

The later is compared to simulated globular cluster systems of known age and size composition in order to set constrains on relative ages and size of the sub-populations.

Keywords:globular clusters: general, galaxies: star clusters, galaxies: individual (NGC 4365, NGC 5846)

4.1 Introduction

Photometric studies on globular cluster systems are of growing interest for our understanding of how and when galaxies form and how they evolve. Being less expensive in observing time than spectroscopy, they are the first step in surveys on the age structure of globular cluster systems. Due to the age-metallicity degeneracy (e.g. Worthey 1994) photometry alone can, in terms of accuracy in the age determination, not compete with spectroscopic data. Nevertheless, single stellar population (SSP) models (e.g. by Maraston 2000; Bruzual & Charlot 1993; 2003, and Vazdekis 1999) allow the detection based on color distributions of globular cluster sub-populations differing in age by several Gyr. With the introduction of near-infrared photometry (Kissler-Patig et al. 2002; Puzia et al. 2002; Hempel et al. 2003; Hempel & Kissler-Patig 2004, hereafter cited as Paper I, II, III and IV) the age resolution improved significantly and relative ages of globular cluster sub-populations in a galaxy could be derived.

An advantage of photometry for these type of work is that it surpasses spectroscopic investi- gations with regard to the observing time required to obtain data for a statistical relevant sample. As we demonstrated in Paper IV, the sample size is a key issue in our method to derive the relative age and size of globular cluster sub-populations. The latter is, as first considered by Ashman & Zepf (1992), linked to the formation scenario of a given galaxy. The accuracy to which the age and metallicity as well as extinction and mass can be determined depends strongly on wavelength coverage of the observations, as shown in a recent work by Anders et al. (2003). With the partly lifted age -metallicity degeneracy in combined optical/near-infrared photometry, the age resolu- tion within an intermediate age population can be further improved by the coverage of the short wavelength range, sensitive to age of integrated stellar populations.

Thus, a more accurate age dating by including U- and/or B-band observations become pos- sible. Also, because a hidden disadvantage of the infrared observations was the limiting magnitude and associated higher photometric errors that needs to be conceded. The latter translates directly into a finite age accuracy which prevents us from taking full advantage of the (V −I) vs.(V −Ks)

color-color diagram. As shown in Figure 4.1, (U −I) improves the age resolution by a factor of

∼ 3 with respect to (V −I) when matched with (V −K) colors. As an example we compare the color difference between a 15 Gyr and a 5 Gyr old cluster population. Hereby we stay in the red color range ((V −Ks)≥2.6), where we expect to find a second generation of metal-rich globular

clusters (Paper III). The (U −I) color between both age populations differ by _∼> 0.4 mag. This is well above the error limit of 0.15 mag for both colors which we typically apply to select reliable photometry for globular clusters.

In the following, we present the results of our U- band observations of two galaxies, NGC 4365 and NGC 5846, combined with previously obtained V, I, and Ks- band data. In both galaxies in-

and IV the (U−I)vs.(V −Ks) color-color distribution will be used to derive the cumulative age

distribution and to set stronger constraints on relative age and size of the second generation of globular clusters. To show the effect of the extended wavelength range on the age distribution we will compare the results for both color combinations (i.e. U V IKs vs.V IKs).

Fig. 4.1.— Both panels show the difference in the secondary color ((V−I), left or (U−I), right) for a 5 (solid circles), 7 (open stars), 10 (solid squares) and 13 Gyr (open triangles) isochrone compared to a 15 Gyr old population (following the Bruzual & Charlot 2000 SSP model ). The metallicity increases with (V −Ks) from 0.005 Z up to 2.5 Z.